Unfortunately they are now dead until you go to new game plus. There are three outcomes to that event. Either they are fully killed (forget what you get if that happens), they live but with less than 50% health (they will die later but will give you a ring, however you miss out on a titanite slab), or they live with more than 50% health (they will fully survive to go onto the next part of the quest, but you receive no ring).

Yeah, he's gone/ she's gone. That part's the most bullshit because even if you save him he usually ends up dying anyway. You can't kill them before hand either.